CMY 383: Spectroscopy (SR) Actual Questions And Correct Detailed Answers.

Describe electromagnetic radiation - correct answer 1. Electron radiations are released as photons 2. Quantized harmonic waves travel at the speed of light 3. Electric and magnetic waves are perpendicular to each other 4. Transition between the states induced by electromagnetic radiation: M + hν → M* Describe the Wave nature of a beam of single-frequency electromagnetic radiation - correct answer 1. a plane-polarized wave is shown propagating along the x-axis. 2. The electric field oscillates in a plane perpendicular to the magnetic field. 3. If the radiation were unpolarized, a component of the electric field would be seen in all planes. Describe Wave Characteristics - correct answer 1. Amplitude (A): of the wave is the length of the electric field vector at the wave maximum 2. Period (p): The time in seconds required for the passage of successive maxima or minima through a fixed point in space 3. The frequency (v): is the number of oscillations of the field that occur per second and is equal to 1/p 4. The wavelength (y^) is the linear distance between any two equivalent points on successive waves (e.g., successive maxima or minima) 5. velocity of propagation (v)i in meters per second: νi x yi Describe the electromagnetic spectrum - correct answer 1. Radiowaves (NMR): Excitation of nuclear spin states, 2. Microwaves: Excitation of rotational energy states 3. Infrared (IR): Excitation of vibrational energy states, i.e. stretching and bending of chemical bonds e.g., organic functional groups. 4. UV-Vis: Excitation of electronic energy states, e.g. π-π* transition Describe polarization of radiation - correct answer 1. Polarized radiation: - a beam of monochromatic radiation can be visualized as an infinite set of electric vectors that fluctuate in length from zero to a maximum amplitude 2. Unpolarized radiation: - electric vectors are viewed at various times during the passage of one wave of monochromatic radiation through a fixed point in space. 3. Plane polarized radiation: - the electric vectors are at the instant the wave is at its maximum. - The vector in any one plane, e.g. XY, can be resolved into two mutually perpendicular components AB and CD - Removal of one of the two resultant planes of vibration produces a beam that is plane-polarized. - The resultant electric vector of a plane-polarized beam then occupies a single plane. Describe the processes happening in a Milikan's photoelectric experiment and the results obtained thereof. - correct answer 1. a vacuum phototube circuit was used to study the photoelectric effect 2. The surface of the large photocathode is usually coated with an alkali metal or one of its compounds. 3. When monochromatic radiation impinges on the photocathode, electrons are emitted from its surface with a range of kinetic energies. 4. As long as the voltage V applied between the anode and the cathode is positive, the electrons are drawn from left to right through the phototube to produce a current I in the circuit. 5. When the voltage across the phototube is adjusted so that the anode is slightly negative with respect to the cathode, the photoelectrons are repelled by the anode, and the photocurrent decreases as expected. 6. At this point in the experiment, however, some of the electrons have sufficient kinetic energy to overcome the negative potential applied to the anode, and a current is still observed. 7. This experiment may be repeated for phototubes with different materials coating the photocathode. 8. In each experiment, the photocurrent is measured as a function of the applied voltage, and the voltage V0 at which the photocurrent becomes precisely zero is noted. 9. The negative voltage at which the photocurrent is zero is called the stopping voltage. 10. It corresponds to the potential at which the most energetic electrons from the cathode are just repelled from the anode. 11. If we multiply the stopping voltage by the charge on the electron, we have a measure of the kinetic energy in joules of the most energetic of the emitted electrons. Milikan's photoelectric apparatus was used to study the photoelectric effect. What are the results obtained when the experiment was repeated for various frequencies of monochromatic light? - correct answer 1. When light of constant frequency is focused on the anode at low applied negative potential, the photocurrent is directly proportional to the intensity of the incident radiation. 2. The magnitude of the stopping voltage depends on the frequency of the radiation impinging on the photocathode. 3. The stopping voltage depends on the chemical composition of the coating on the photocathode. 4. The stopping voltage is independent of the intensity of the incident radiation. 5. These observations suggest that electromagnetic radiation is a form of energy that releases electrons from metallic surfaces and imparts to these electrons sufficient kinetic energy to cause them to travel to a negatively charged electrode. 6. Furthermore, the number of photoelectrons released is proportional to the intensity of the incident beam. Maximum kinetic energy or stopping energy formula - correct answer 1. KEm = eV0 2. KEm = hv - w 3. and E = hv What are the (two) postulates of quantum theory while considering transition between two energy states. - correct answer 1. Atoms, ions, and molecules can exist only in certain discrete states, characterized by definite amounts of energy. - When a species changes its state, it absorbs or emits an amount of energy exactly equal to the energy difference between the states. 2. When atoms, ions, or molecules absorb or emit radiation in making the transition from one energy state to another, the frequency v or the wavelength y of the radiation is related to the energy difference between the states by the equation: - E1 - E0 = hv = hc/ y - where E1 is the energy of the higher state and E0 the energy of the lower state. - The terms c and h are the speed of light and the Planck constant, respectively. Explain emission or chemiluminescence processes and indicate the transitions and spectral outputs thereof (draw) - correct answer 1. the analyte is stimulated by heat or electrical energy or by a chemical reaction. 2. Emission spectroscopy usually involves methods in which the stimulus is heat or electrical energy, and chemiluminescence spectroscopy refers to excitation of the analyte by a chemical reaction. (These processes do not involve radiant energy and are hence called nonradiative excitation processes.) 3. Prior to applying the stimulus, the analyte is predominantly in its lowest energy state, or ground state. - The stimulus then causes some of the analyte species to undergo a transition to a higher energy, or excited state. - the dashed black lines with upward-pointing arrows symbolize nonradiative excitation processes, while the solid colored lines with downward-pointing arrows indicate that the analyte loses its energy by emission of a photon. 4. In both cases, measurement of the radiant power emitted (PE) as the analyte returns to the ground state, can give information about its identity and concentration. 5. The results of such a measurement are often expressed graphically by a spectrum, which is a plot of the emitted radiation as a function of frequency or wavelength. Explain absorption processes and their spectral outputs. (draw) - correct answer 1. sample is stimulated by application of an external electromagnetic radiation source 2. Radiation of incident radiant power P0 can be absorbed by the analyte, resulting in a transmitted beam of lower radiant power P and promotes some of the analyte species to an excited state. 3. For absorption to occur, the energy of the incident beam must correspond to one of the energy differences. 4.In absorption spectroscopy, we measure the amount of light absorbed as a function of wavelength. Explain photoluminescence processes and their spectral outputs (draw) - correct answer 1. Fluorescence and phosphorescence result from absorption of electromagnetic radiation and then dissipation of the energy emission of radiation 2. the absorption can cause excitation of the analyte to state 1 or state 2. 3. Once excited, the excess energy can be lost by emission of a photon (luminescence, shown as solid line) or by nonradiative processes (dashed lines). 4. The emission occurs over all angles, and the wavelengths emitted correspond to energy differences between levels. 5. The major distinction between fluorescence and phosphore